This invention relates a method of automatically controlling chemical concentration in a metal strip cleaning line, particularly an aluminum strip cleaning line.
In the processing of aluminum strip, e.g. for use in automotive production, it is necessary to clean the surfaces of the strip material. One way of doing this is by passing the strip material on a continuous basis through a cleaning line which includes an acid wash section or sections(s), followed by a rinse section or sections. In each section acid solution or rinse water respectively is sprayed via nozzles onto the top and bottom faces of the strip passing through the line. The sprayed liquid is collected in tanks at the bottom of the enclosure from where it is re-circulated by pumps back through the nozzles.
In this procedure it is important to control the free acid strength in the wash section(s), and this is typically done by providing a conductivity probe immersed in the fluid in the bath. A specific conductivity signal which varies approximately in proportion to the free acid concentration of the solution is typically provided by the conductivity probe and this is used to adjust the acid concentration. However, use of the specific conductivity signal to estimate the free acid concentration is prone to offset errors due to impurities, and temperature fluctuations in the bath which increase or decrease the actual specific conductivity of the cleaning solution independently of free acid concentration, and errors due to drift of the conductivity probe itself caused by build up on the electrode.
Because of this unreliability, it is the usual practice to periodically perform manual titrations to verify that the concentration is still within limits. If a discrepancy is found, options are manual recalibration of the probe or manual adjustment of the bath using trial and error methods. This requires skilled technicians and/or operators, is labour intensive and is subject to operator errors in the calibration and correction. It has been found that the bath concentration can experience considerable drift from target before a correction is made, resulting in product which is improperly processed.
In making the above corrections, the objective is to control the free acid concentration (FAC) in the bath, which is the acid available for reaction with the aluminum surface as opposed to the total acid concentration (TAC). The total acid concentration comprises the free acid concentration plus soluable reaction products. Control of the free acid concentration is done by estimating the free acid concentration and adding fresh concentrated acid from a storage tank or water depending on whether the free acid is too low or too high.
Japanese Patent Publication JP 7-54175, published Feb. 28, 1995 describes a method of controlling acid concentration in a steel pickling line by monitoring weight loss. However, it is not concerned with the problems related to a cleaning line and the importance of the correctness of the free acid concentration.
It is also known to clean metal strips by passing the strip through a cleaning line where an alkali solution is used rather than an acid solution. An example of this can be found in Japanese Patent Publication JP 11-269,678, published Oct. 5, 1999, where an alkali solution was used to degrease and clean cold-rolled steel strips. When cleaning with alkali solution, the same problems in controlling concentration are encountered as described above for acid cleaning solutions.
It is the object of the present invention an automated and more accurate method of controlling the chemical concentration in a metal strip cleaning line.
One embodiment of this invention relates to a method for automatically controlling acid concentration in an aluminum strip cleaning line in which an aluminum strip is contacted with an acid solution while passing through a cleaning bath and the concentration of the acid in the bath is adjusted by adding either concentrated acid or water to the bath. A conductivity probe is provided in the acid cleaning bath and this probe generates a first signal approximately proportional to the free acid concentration of the bath. An on-line process titrator is also provided to periodically sample the acid bath and by a dual endpoint titration, obtain the free acid and total acid concentration of the bath. The on-line titrator generates a second signal indicative of the actual free acid concentration and the total acid concentration. The above signals are fed to a programmable logic controller (PLC) which, based on the signal from the titrator, calculates a correction factor for the signal from the conductivity probe to thereby obtain a corrected specific conductivity value proportional to the free acid concentration. Based on this corrected value, the bath acid concentration is automatically and continuously adjusted as required based on the corrected specific conductivity value. The acid used for this purpose is typically sulfuric acid. The difference between the total and free acid concentration is indicative of the level of bath contaminants and can therefore be used to adjust the amount of metered overflow from the wash section.
The conductivity probe measures the ability of a solution to conduct an electric current between two electrodes. An increase in concentration of ions in the solution results in higher conductivity values. Conduction is measured in Siemens (formerly known as mho) and the conductivity probe can also be used to find the concentration of total dissolved solids in a sample of water.
A new titration is conducted automatically at timed intervals or when requested by the PLC and the value of the specific conductivity at the time of the titrator sample being drawn is stored in the PLC memory. The titration timer is reset to zero after each successfully completed titration.
The titrator free acid titration % input validation is accomplished as follows. If the difference between the new value of free acid and the recorded value of free acid is more than x percent of the current value, the current value is retained and a new titration is requested by the PLC. A warning signal is sent to the operator station advising xe2x80x9cAcid Concentration out of Rangexe2x80x94Rechecking Concentrationxe2x80x9d. This is to ensure that any large discrepancies are not due to a titration error or anomaly. The value of the next titration is accepted by the PLC and replaces the current value.
Immediately upon receiving a valid reading of free acid from the titrator input, a Conductivity Correction Factor is calculated and stored in the PLC. The calculation is as follows:
Conductivity Correction Factor=Free acid Titration % xe2x88x92{Specific conductivity (xcexcSiemens/cm)*Concentration Factor (1/xcexcSiemens/cm)}
The Concentration Factor is the conversion factor for specific conductivity to Free Acid % with units 1/xcexcSiemens/cm which is determined once for each type of cleaning solution by an off-line calibration. The true acid concentration based on the output of the conductivity meter is determined in the PLC by the following algorithm:
xe2x80x83Conductivity Corrected Free Acid Concentration %={Specific Conductivity (xcexcSiemens/cm)*Concentration Factor (1/xcexcSiemens/cm)}+Conductivity Correction Factor
A value indicative of the level of bath contamination is also calculated in the PLC as follows:
Contaminant Level=Total acid Titration %xe2x88x92Free acid Titration %
The acid cleaning bath is typically followed by one or more rinsing sections where the acid cleaned strip is thoroughly rinsed. The cleaning and rinsing liquids are preferably sprayed on the strip material by means of a plurality of spray nozzles above and below the strip. These are connected to pumps to simultaneously spray the top and bottom faces of the moving strip. The cleaning and rinsing liquids flow back down into a reservoir in each section to be re-circulated through the nozzles.
In a further embodiment of the invention, the acid cleaning bath in the above description may be replaced by an alkali cleaning bath. The same procedures as described above are then used to control the alkali concentration in the cleaning bath.
The method of this invention has the important advantage of requiring no skilled technicians or operators and allowing more precise control of the process by semi-continuously compensating for measurement errors.